Did anybody mention tractor-trailer gap management already?
You want half- cone or rounded edges on front side of your trailer, unless you are filling the gaps with stretched spandex. This prevents vortex generation in the gap.
Somewhere else on the forum it was already discussed, halves of large diameter sewage pipes should suffice iirc.
EDIT: I found the thread:
http://ecomodder.com/forum/showthrea...tml#post502508,
http://ecomodder.com/forum/showthrea...ent-33247.html
original image posted by aerohead